1. Field of the Invention
The present invention relates to a transmitting technology, and it particularly relates to a transmitting method and apparatus for performing MIMO eigenmode transmission.
2. Description of the Related Art
One of techniques to realize a higher quality and a higher data transmission rate in a wireless communication system is a MIMO (Multiple-Input Multiple-Output) system. In this MIMO system, a transmitting apparatus and a receiving apparatus are each equipped with a plurality of antennas, and a plurality of channels corresponding respectively to the antennas are set. Accordingly, channels up to the maximum number of antennas are set for the communications between the transmitting apparatus and the receiving apparatus so as to achieve a high data transmission rate. Of such MIMO systems, a MIMO eigenmode system can increase the channel capacity. In the MIMO eigenmode system, derived is a channel matrix (hereinafter referred to as “H matrix”) which is generated from values of channel characteristics between a plurality of antennas provided in the transmitting apparatus and those in the receiving apparatus. Then, eigenbeams corresponding to orthogonal channels the number of which is equal to the rank of an H matrix are formed in the MIMO eigenmode system. In so doing, the eigenbeams corresponding respectively to the orthogonal channels are formed.
When the thus formed eigenbeams continue to be fixed, the antenna directivity and the side lobe also continues to be fixed. In such a case, there are cases where radio apparatuses, which are not targeted ones, can continue to receive the eigenbeams at a certain level of signal strength. As a result, continuous interception is likely to occur. In terms of security, it is desirable that untargeted radio apparatuses will not continue to intercept.